Breakthrough Starshot: Focus on the Sail
Who knows why and when we’re going to remember things? In the bus on the way to Moffett Field for the second morning of the Breakthrough Starshot meetings, I found myself thinking about Poul Anderson’s The Enemy Stars (1959). I had a paperback edition with a beautiful Richard Powers cover when I was a boy. What haunted me on that drive was the memory of what was written on the back:
They built a ship called the Southern Cross and launched her to Alpha Crucis. Centuries passed, civilizations rose and fell, the very races of mankind changed, and still the ship fell on her headlong journey toward the distant star. After ten generations the Southern Cross was the farthest thing from Earth of any human work – but she was still not halfway to her goal.
Breakthrough Starshot doesn’t plan to take that long to reach one of the Alpha Centauri stars (Alpha Crucis, by the way, is not one of them, but a multiple star system that is a part of the beautiful asterism known as the Southern Cross). The immensity of a journey between the stars still astounds me, after all these years of writing about it. At Moffett Field’s Building 18, we would be talking about ways to cross such gulfs. And while the long result is always at the back of your mind, it was time here in the Bay area to start talking about what can be done soon.
I have discovered that bilocation — being in two places simultaneously — is impossible. In the meetings at Moffett Field, I wanted to listen and take notes in each of the subcommittees, but stayed with just one, the sail group, because it seemed the best way to get a sense of the process as it worked itself out over the three days. Tough choice, because my respect for laser group leader Bob Fugate (New Mexico Tech) is immense, and I wanted to see how he and his team would deal with the early conceptualization of an unthinkably vast laser beamer. I also wanted more of the overview that Kevin Parkin’s systems engineering group was constructing.
Fortunately, our sessions frequently coincided as the subcommittees reported back to the full group, and it was possible to keep up to speed. The days were long, the debate fast-paced and productive, taxing my powers as a note-taker. But something Greg Benford said in the early going kept resonating with me throughout the meetings. “Nature bats last,” Greg commented after explaining his thoughts on beaming energy to a sail. In other words, we can produce idea after idea, but ultimately they’re going to be tested in simulations and in the laboratory, and we’re going to wind up with what works, not necessarily with our preconceptions.
Image: Did I mention that it was crowded when the full group met? This is from the first day of meetings, with several of the people from Breakthrough Listen also on hand to report on their own sessions.
Working on the Sail
We know what the long-term plan is: To develop and build a system that can deliver a payload to the Alpha Centauri system, said system to be perfected within a 20 to 30 year time frame, followed by 20 years of interstellar flight and a data return period of another 5 years. The plan is to send not one but many of these probes. In fact, if we are able to build a suitable beamer, we’re creating a reusable deep space infrastructure. Now, sitting in the board room at Moffett Field with afternoon sunlight slanting across the table, the sail committee was talking about the ‘short’ run, a 5-year period of technology development designed to lead to a prototype.
Technology development involves deep study of the concept, followed by simulations leading to laboratory work, and we were helped in the early sail discussions by the fact that Jim and Greg Benford, along with the University of New Mexico’s Chaouki Abdallah, had already performed laboratory work on microwave beamed sails using lightweight, highly temperature resistant carbon fiber. Out of this work’s simulations and experiments, the 4-meter sails envisioned for Breakthrough Starshot can grow conceptually and with new rounds of experimentation.
We have a sail that must achieve 20 percent of lightspeed and survive an intense period of acceleration lasting merely minutes. This is a craft that must be able to operate more than 20 years, given travel time and data return, and the sail that carries the payload must demonstrate stability on the beam, meaning that the slightest imperfection in design could cause it to simply be flung off-course. Moreover, the sail must be readily deployable, and it must be able to withstand the rigors of launch from Earth. The choice of materials for the sail flows directly from these requirements. Did I mention that we need to use the sail to return data to Earth? And that dust particles in the interstellar medium are a serious issue, though thought manageable?
Image: Work going on even during the breaks. This is mostly the sail committee, with Jim Benford at left, Breakthrough Starshot executive director Pete Worden, Greg Benford, Rafael Fierro, and Greg Matloff. I believe that’s Roald Sagdeev just behind Rafael.
We know that a beamed sail can be stable under a beam if it is spinning — the Benford/Abdallah lab studies have already demonstrated this (bringing the sail up to its initial spin, which involves sail deployment issues, is a key requirement). New rounds of simulation will be looking at matters like the best sail shape. A spinning conical shape is preferred, with the payload distributed in the lower part of the sail, which must be built so that a net sideways force tends to restore the sail onto the beam rather than pushing it off. The spin imparted to the sail comes from initial deployment, as previously demonstrated in tests at JPL. The sail becomes stabilized against pitch and yaw, but the center of force must always be above the center of mass.
This last, by the way, is why a rocket is not stable. The center of force is actually below the center of mass in this case, a situation we don’t want to see on the sail, which is why the payload cannot be placed on top of the sail. And given the power levels the team plans to put onto the sail, we have to identify issues that have so far escaped us. “In terms of beam-riding and stability,” Jim Benford noted, “we are going to learn new problems we have not thought of yet. That’s why we have to get to experiments sooner rather than later.”
The sail committee debated these matters much of the afternoon on the first and second day, a team involving Lou Friedman (Planetary Society), Mason Peck (Cornell), Starshot director of engineering Pete Klupar (who would move between sail and laser meetings), Kelvin Long (i4IS), Zac Manchester (Harvard), Raphael Fierro (UNM), Greg Matloff (CUNY), Chaouki Abdallah (UNM) and both Benfords. Jim Benford, who chaired the sail meetings, pointed to the need for experiments both vertical and horizontal, the vertical to test stability and spin, the horizontal to test acceleration.
To get moving quickly, the sail committee decided to identify and contact industrial, academic or governmental research groups that can be of help, with a workshop on sail materials as soon as the end of September. Work on simulations can likewise begin quickly, with sails tested in a vacuum under a variety of accelerations. If the laser for this early testing cannot be provided in that timeframe, microwaves can be a stand-in, with a variety of advantages of their own. Meanwhile, the analysis, simulation and fabrication will involve multiple contracts, with down-selection at the end for the best solution to the complicated sail requirements.
As I mentioned the other day, a Request for Proposal (RFP) is already being drafted — we worked on this the last day of the meeting, at the hotel. Results on that should begin to emerge in the spring of 2017, even as Breakthrough Starshot begins trade studies that will help in the evaluation of these proposals. The RFP will help to select the people who can do the first round of studies. Stability testing on the sail can begin relatively soon, Jim Benford said, while developing the requirements for acceleration will depend on chosen contractors.
Image: Late in the morning of the last day, the sail committee, consulting at the hotel with laser subcommittee leader Bob Fugate. From left, Greg Benford, Mason Peck, Zac Manchester. I think that’s Wes Green (Tau Technologies) just behind Zac, then Jim Benford, and Bob Fugate. Not visible at the end of the table: Greg Matloff and Rafael Fierro.
Deep Space Reminiscences
With Anderson’s The Enemy Stars still bouncing around in my head, I joined the Starshot team for an evening at a local restaurant. I was fortuitously placed directly across from Olivier Guyon (University of Arizona) and Slava Turyshev (JPL), which led to conversations about all of us and how we got involved at an early age in space. I always ask the scientists I talk to about this, and it invariably generates discussions about long-remembered favorite books and movies. Few of us have just one key driver, and I can recall films like Destination Moon as well as the Anderson novel among numerous other books and short stories.
Guyon recalled the Fantômas books, a long series by various French authors involving wild plots and inventive gadgetry — with an anti-hero main character to boot. Turyshev had read some of these in the Soviet Union and had even built his own versions of some of the unusual technologies, like a flying car with retractable wheels. Novels were my own introduction, especially the Heinlein juveniles, and Jim Benford jumped in with his own reminiscences of such titles as Have Spacesuit Will Travel (for me, it was Starman Jones). Amidst all this, Ed Turner (Princeton) and I squeezed in a talk about his travels in Japan.
Conversation flows easily after days of data crunching and analysis (wine doesn’t hurt, either). But the size of the project was something that stayed with me throughout these meetings, and it was only reinforced by the memory of Heinlein and science fiction’s Golden Age. Walking back to the hotel on a fine Palo Alto evening with Claire Max and Kevin Parkin, I found myself pondering the audacity of a star mission. “They built a ship called the Southern Cross and launched her to Alpha Centauri…” Name aside, it’s the plan, though the ‘ship’ is nothing like Anderson’s. It’s small, sail-driven, and can be sent out in swarms. It will need to travel 4.2 light years, fully 260,000 AU (if Proxima is the target, another 10,000 AU or so if it’s Centauri A or B). And what put me in an irresistibly light mood was the thought that the physics does not preclude it. The engineering, though, is another matter, and a key focus of all these deliberations.
